Large two-stroke internal combustion engine

ABSTRACT

An internal combustion engine (1) has hydraulically driven exhaust valves (13) and fuel pumps (18). The hydraulic drives are controlled by means of a computer (16) and electrically activated positioning means (64) setting a spool in a spool valve. If the electronic control of the engine fails, the spool movement may be controlled by a first piston (41) on which the pressure in a hydraulic hose or conduit (48) acts, said conduit extending to a second piston (44) which may follow a cam (26) on a rotating camshaft. The hydraulically driven cylinder members (13, 14, 18) associated with each of the engine cylinders are mounted at the pertaining cylinder, whereas the camshaft (23) independently of the positioning of the cylinder members is disposed at an appropriate shaft drive, such as the crankshaft (11). The cam shaft has a very short length and small mass and may for instance be disposed at one end of the engine.

The invention relates to a large two-stroke internal combustion engine,in particular a main engine of a ship, having a hydraulically drivencylinder member, such as a fuel pump or an exhaust valve, in which thehydraulic drive of the member comprises a driving piston journalled in ahydraulic cylinder which, through a flow passage, communicates with aspool valve, the spool of which may occupy a position where the flowpassage communicates with a high pressure source for hydraulic oil, andanother position, where the flow passage communicates with a lowpressure port, where during normal engine operation, the spool ispositionable by means of an electrically activated positioning meansreceiving control signals from an engine controlling computer, andwhere, in case of failure of the normal engine control, the spool isalternatively positionable by means of a camshaft rotating synchronouslywith the crankshaft of the engine.

Such an internal combustion engine is known from for exampleinternational patent publication No. WO89/03939, where the camshaft isof the conventional type whose cam acts directly on a rod connected withthe spool or acts on a secondary spool mounted on the spool housing. Thepublication also indicates that between the cam and the rod connectedwith the spool, a transversely movable rod may be inserted having anidler contacting the cam, which makes it possible to change the timingof the cam action on the control spool.

In the known engines, the camshaft is positioned immediately below thecylinder members to be activated by the cams. The camshaft extends inthe full longitudinal direction of the engine to be able to act on thecylinder members of all the cylinders. In consequence of its length, thecamshaft has a large mass and is relatively expensive to manufacture,just as it uses a deal of energy, as it participates in the movements ofthe crankshaft. To ensure a synchronous movement of the camshaft inrelation to the crankshaft, the two shafts are connected by means of achain drive, which may have a mass of several tonnes in a large internalcombustion engine. The bearings and cams of the camshaft further have tobe lubricated, which requires designing of oil ducts and lubricating oilpumps, etc., for the camshaft.

The purpose of the invention is to simplify the engine by providing asmall camshaft which may be mounted at a distance from the cylindermembers activated by the camshaft.

With this in view, the internal combustion engine according to theinvention is characterized in that the spool is associated with a firstpiston on which the pressure in a hydraulic conduit acts, said conduitextending to a second piston which may follow a cam on the rotatingcamshaft, and that the hydraulically driven cylinder members associatedwith each of the engine cylinders are mounted at the pertainingcylinder, whereas the camshaft independently of the positioning of thecylinder members is disposed at an appropriate shaft drive, such as thecrankshaft.

The spool valve only requires a relatively small force to activate thehydraulically driven cylinder member, which permits the hydraulic hoseor conduit interconnecting the first and the second piston to have sucha small internal diameter that the amount of hydraulic oil in theconduit will not be very large, even if the conduit is of great length.It is therefore possible to obtain an accurate transmission of themovements of the second piston to the first piston, even though thecamshaft is positioned at a large distance from the cylinder members.The hydraulic conduits with the associated pistons act as a rigid pushrod, even though there is a vertical and horizontal distance of manymeters between the positions of the first and the second piston. Thehydraulic force transmission between the two pistons associated witheach cylinder member therefore permits the camshaft to be disposed atany suitable shaft drive. It is, for example, possible to position thecamshaft at the end of the engine in direct toothed engagement with thecrankshaft. The camshaft may also be disposed as an extension of theshaft driving the cylinder lubricating devices. All the pistons drivenby the camshaft with associated connections for the hydraulic conduitsmay be arranged closely next to each other in a single unit, so that thecamshaft has an extremely short length and thus small mass. The energyconsumption for driving the camshaft will therefore be a minimum andquite negligible in relation to the total energy consumption of theengine, which increases the efficiency of the engine. The previouslyknown large chain drive and the elongated housing for the camshaft alsocompletely disappears, which gives a marked reduction of the totalweight of the engine and makes the manufacture of it cheaper.

As the camshaft with the associated hydraulic push rods is only amechanical emergency control system for use in case of failure in theelectronic engine control, during normal engine operation the firstpiston is preferably prevented from transmitting the cam movement to thespool, whereby the spool and the electronic control system remainuninfluenced by the mechanical emergency control system during normalengine operation.

With a view to reducing the energy consumption of the engine, but at thesame time keep the mechanical emergency control system ready forimmediate operation, a preferred embodiment is characterized in that thesecond piston is lifted free of the camshaft when the engine control isnormal, and that the second piston is brought into contact with a cam onthe camshaft, when the latter is to be engaged. During normal operation,the camshaft is thus uninfluenced by the second piston associated witheach cylinder member, so that no energy is delivered to the hydraulicconduits interconnecting the first and the second pistons. The firstpiston for each cylinder member thus stands still during normal engineoperation and thus cannot transmit cam movements to the spool. Liftingthe second piston off the camshaft renders it possible to keep thehydraulic conduit between the two pistons filled with hydraulic oil, sothat the emergency control system may be engaged in a fraction of anengine cycle, if a failure occurs in the electronic engine control.However, as an alternative to the lifting off of the second piston, itis possible to deactivate the camshaft control by opening a puncturevalve in the hydraulic conduit, but this involves a risk of airpenetrating into the hydraulic conduit, which will destroy an accuratecamshaft control.

The amount of oil in the hydraulic conduits may further be reduced byadapting the spool to follow the movements of a small pilot spool whichis controlled during normal operation by the electrically activatedpositioning means and alternatively by the movements of the firstpiston. The force needed for setting the pilot spool is substantiallysmaller than the force for setting the spool which regulates the oilflow to and from the driving piston, and the use of a pilot spool thusrenders it possible for the first and the second piston to be given verysmall dimensions, and for the internal diameter of the hydraulicconduits to be only a few millimeters. This contributes towards makingthe amount of oil in the hydraulic conduit so small that the hydraulicpush rod becomes very fast-acting and has a very small energyconsumption. The mechanical action of the second piston on theassociated cam also becomes very slight, and thus the camshaft may bedesigned with very small dimensions.

A structurally particularly simple embodiment is characterized in thatthe pilot spool is positioned coaxially inside the spool and is fastenedto a rod which is rigidly connected to the movable part of thepositioning means and projects to one side of the spool, and that thefirst piston is positioned to the other side of the spool and carries arod which extends coaxially with the spool to the pilot spool.

To prevent any contact during normal engine operation between theemergency control and the pilot spool, the first piston with theassociated rod is suitably spring-loaded for movement away from thepilot spool. The spring loading also ensures an accurate return of thefirst piston, when the camshaft control is activated, and the secondpiston follows a declining cam profile.

Preferably, the movable part with associated rod of the positioningmeans is spring-loaded for movement towards the first piston, and duringnormal engine operation the positioning means overcomes the springloading. In case of failure in the electronic engine control, the springloading of the movable part of the positioning means results in thepilot spool immediately being pushed over to abut on the rod connectedwith the first piston, so that the camshaft immediately takes over thecontinued engine control. If, before the failure of the electroniccontrol, the second piston abuts on the camshaft, the engine will besubstantially unaffected by the failure. In the cases where the secondpiston first has to be brought into abutment with the associated cam,the engagement of the emergency control will be delayed by theengagement time of the piston.

Owing to the short length of the camshaft, the hydraulic conduits forthe cylinder members of the different cylinders have a varying length.The oil in the hydraulic conduits has a certain absolute compressibilitydepending on the amount of oil in the conduits. If the conduits containdifferent amounts of oil, the camshaft movement will be transmitted mostrapidly to the first piston of the conduits which contain least oil,i.e. the short conduits. It is possible to compensate for this byturning the cams associated with the short conduits a little back on thecamshaft, but it is simpler to design the engine so that at least someof the piston-connecting hydraulic conduits leading to the same kind ofcylinder members are in communication with a respective compensatingvolume of a size so that the hydraulic conduits contain a substantiallyequal amount of hydraulic oil.

The camshaft has to be able to control the engine, both during forwardrunning and reverse running. As the fuel injection and the opening ofthe exhaust valve are normally not initiated when the piston is exactlyin its top dead centre position, but is displaced a few degrees inrelation to this, a cam timed for running forward will not give thecorrect timing in case of reverse running. From the above internationalpatent application it is known that the timing may be changed bydisplacement in relation to the cam of an idler mounted on atransversely movable rod. A suitable further development of this priorart is characterized in that in its active position, the second pistonabuts on the upper side of a rod which on its lower side carries anidler contacting the associated cam, that the rod is transverselymovable in relation to the longitudinal direction of the camshaftbetween an extreme position for use during running of the engine in thenormal direction of rotation, and another extreme position for useduring running of the engine in the opposite direction of rotation.

By letting the rod be movable between two extreme positions for use atforward running and reverse running, respectively, the rod may becontrolled in a very simple manner, for example by means of acompressed-air cylinder forcing the rod to be either in one or the otherextreme position. To obtain the correct timing of fuel pumps and exhaustvalves it is thus only necessary to shift a single control valve for thepneumatic cylinder.

The two extreme positions of the rod are suitably adjustable, so thatthe timing may be adjusted in relation to the actual engine load. Theextreme positions may, for example, be fixed by means of two manuallyadjustable, mechanical stops. In case of operation of long duration at acertain engine load, the operating staff may adjust the stops by meansof an instruction showing the relationship between the engine load andthe optimum position for the stops.

An example of an embodiment of the invention will be described infurther detail below with reference to the very schematic drawings, inwhich

FIG. 1 shows an outline of an internal combustion engine,

FIG. 2 is a diagram of the hydraulic connections to an emergency controlsystem for the engine,

FIG. 3 is a side view of a camshaft for the engine of FIG. 1,

FIG. 4, on a slightly larger scale, an end view of the camshaft shown inFIG. 3 with associated equipment for adjusting the timing,

FIG. 5 is a longitudinal sectional view through a spool valve for acylinder member, and

FIG. 6, on a larger scale, a segment of the spool valve of FIG. 5.

FIG. 1 shows a large two-stroke diesel engine of the crosshead typegenerally designated 1, which may be used as the main engine of a shipor as a stationary power-producing engine. The combustion chamber 2 ofthe engine is delimited by a cylinder liner 3 and a cylinder cover 4 anda piston 5 journalled in the liner.

Via a piston rod 6, the piston is directly connected with a crosshead 7which, via a connecting rod 8, is directly connected with a connectingrod pin 9 in a throw 10 of a crankshaft 11. A cylinder member in theform of an exhaust valve 12 with associated housing 13 is mounted on thecover 4. The exhaust valve is activated by a hydraulic drive 14controlled by an electro-mechanical valve activated by control signalstransmitted through a wire 15 from a computer 16.

A fuel valve 17 mounted in the cover 4 may supply atomized fuel to thecombustion chamber 2. Another cylinder member in the form of a fuel pump18 is controlled by an electro-mechanical valve and may supply fuel tothe fuel valve through a pressure conduit 19 in dependency of controlsignals received from the computer 16 through a wire 20. Through asignal-transmitting wire 21, the computer 16 is supplied withinformation on the current number of revolutions per minute of theengine. The number of revolutions may either be taken from thetachometer of the engine, or it may originate from an angle detector andindicator mounted on the main shaft of the engine and determining thecurrent angular position and rotating speed of the engine for intervalsconstituting fractions of an engine cycle of a shaft rotation of 360°.When the computer has determined the time for the fuel injection and theassociated amount of fuel, and the opening and closing times of theexhaust valve, the fuel pump 18 and the drive unit 14 are activatedaccordingly at the moment of the engine cycle which is correct for thecylinder. The engine has several cylinders which are all equipped in theabove manner, and the computer 16 may control the normal operation ofall cylinders.

As explained below, the oil inflow and outflow for the hydraulic drivesof the cylinder members are controlled by a spool valve (or shuttle orslide valve), which is set during normal engine operation by anelectrically activated positioning means reacting on control signalsfrom the computer 16. If, for some reason, a failure occurs in theelectronic control system, the setting of the spool (or shuttle orslide) is taken over by a camshaft control system. This control systemcomprises a camshaft unit 22 with a camshaft 23 rotating synchronouslywith the crankshaft 11 of the engine, for example, by the two shaftsbeing in mutual engagement through two cogwheels 24 and 25. The camshaftunit may be disposed at the end of the engine, but may also, asindicated, be disposed at a suitable place inside the engine. If it isnot desired that the camshaft unit is in immediate proximity to thecrankshaft, the synchronization of the camshaft may alternatively beprovided via a chain or belt drive.

The camshaft unit will now be described in further detail with referenceto FIGS. 2-4. The camshaft unit shown is intended for an engine withfour cylinders, each having two hydraulically driven cylinder members.Thus, the camshaft has eight cams 26 in close proximity to each other,so that the shaft has a short length. As a consequence of the small sizeof the camshaft, it is sufficient to journal it in two bearings 27carried by the camshaft housing 28. By means of a belt pulley 29 and atoothed belt 30 the camshaft is driven synchronously with thecrankshaft. The camshaft is enclosed by a protective casing 31. Theforces acting on the camshaft are so small that the bearings 27 needonly be grease-lubricated, and the cams on the shaft can do withoutlubrication. The previously known camshaft lubricating systems may beomitted completely.

The timing of each cam 26 in relation to the engine cycle takes place bymeans of a rod 33 which abuts the cam periphery via an idler 34. At theend away from the shaft, the rod 33 is journalled on an upright top-hungintermediate rod 35 which, at a distance from its upper journallingpoint, is connected with a piston rod 36 in a pneumatic cylinder 37. Thecylinder 37 may move the intermediate rod 35 and thus the rod 33 betweentwo extreme positions determined by two stops in the form of a set screw38 and an eccentrically journalled disc 39. The extreme positions aresettable by turning the screw 38 and by turning the disc 39 about itsfulcrum 40, respectively. Adjustment of the extreme positions leads to achange of the point of contact of the idler 34 on the cam 26, wherebythe raising and lowering of the rod 33 produced by the cam is phasedisplaced in relation to the rotational movement of the camshaft. In theextreme position shown, with the intermediate rod 35 abutting the setscrew 38, the camshaft unit is set for forward running, while thecamshaft unit with the intermediate rod 35 abutting the disc 39 isintended for reverse running.

When the camshaft control is active, a first piston 41 acts on the spoolof the spool valve of the associated cylinder member. The piston 41 isjournalled in a small hydraulic cylinder 42 mounted at the end of thespool valve housing 43.

The movements of the first piston are controlled by a second piston 44journalled in a small hydraulic cylinder 45 in the camshaft unit. Theend surface 46 of the first piston and the end surface 47 of the secondpiston are in direct contact with the oil in a hydraulic conduit 48, thetwo ends of which are connected to the cylinders of the first and thesecond piston, respectively. The hydraulic pressure hose or conduit 48is bendable and flexible which makes its installation very easy. Theflexibility of the hydraulic conduit 48 permits the camshaft unit 22 tobe disposed at a large distance from the hydraulically driven cylindermembers both in the horizontal and the vertical direction, as roughlyoutlined in FIG. 1 by the dotted lines 48. To obtain an accurate anduniform transmission of the movement of the second piston to the firstpiston, it is important that the amount of oil in the conduit 38 isconstant, and that the conduit is filled all the time.

The oil for the camshaft control may suitably be taken from a pressureconduit 49 supplying high-pressure hydraulic oil to the hydraulic drivesof the cylinder members. As the pressure of this conduit is at about 300bar, the pressure is reduced in an adjustable pressure reducing valve 50to about 10-15 bar, which is fully sufficient to ensure an accuratetransmission of the movements of the pistons. Via a pressure conduit 51,the oil drain of the pressure reducing valve communicates with a valve52 which may occupy two positions. In the active position shown in FIG.2, the conduit 51 is connected to a conduit 53 leading to a pressurechamber 54 on the upper side of a lifting piston 55 which is presseddown at the bottom of the chamber 54 so that a projecting collar on thepiston 44 is positioned at a distance from the upper side of the piston55. The oil pressure in the conduit 48 presses the second piston 44 anda pressure rod 56 rigidly connected with it down for abutment againstthe upper side of the rod 33, so that the second piston is forced toclosely follow the cam profile. Simultaneously, the valve 52 keeps apressure chamber 57 on the lower side of the lifting piston 55 inconnection with a drain 58 via a conduit 59, 59a. The piston 44 and thepressure rod 56 suitably have the same diameter, so that the pressure inthe chamber 54 does not yield any resulting force on the projectingcollar of the piston 44.

The camshaft unit may be deactivated by switching the valve 52 so thatthe pressure chamber 54 is put into communication with the drain 58, andthe pressure chamber 57 is put into communication with the pressureconduit 51, the result of which is that the second piston withassociated pressure rod 56 is lifted free of the cam 26, because thelifting piston 55 is moved upwards in the chamber 54 and hits the lowerside of the collar on the piston 44, whereupon the piston participatesin the upward movement of the lifting piston. A branch conduit 62debouching above the piston 44 is put into connection with the pressurechamber 57 at the valve switching, so that the lifting of the secondpiston 44 does not influence the position of the first piston 41.Simultaneously with the lifting, the rod 33 is lifted free of the cam bymeans of a spring 60. When the chamber 54 is pressurized, the downwardforce on the pressure rod 56 is far greater than the spring load on therod 33.

By a spring 61, the valve 52 is preloaded to the position where thecamshaft control is disengaged, to ensure that the second piston 44 doesnot come into engagement with the cam after a standstill of longduration. A nonreturn valve 63 ensures that the hydraulic conduit 48with associated conduits and pressure chambers 54, 57 is always keptfilled with oil.

FIG. 5 shows how the first piston 41 with associated cylinder 42 ismounted at the end of the spool valve housing 43, which is composed ofseveral pieces bolted together, viz. a central piece and two end covers,where the first piston is mounted in one end cover, while anelectrically activated positioning means 64 is mounted on the other endcover.

The central piece of the housing has a fluid inlet conduit 65communicating with the high-pressure conduit 49, two fluid drainconduits 66 communicating with a low-pressure port, and two outletconduits 67 leading to a pressure chamber 68 in a hydraulic cylinder 69for the hydraulic drive driving the cylinder member. A hydraulic piston70 in the drive is driven upwards by the oil pressure in the chamber 68when the latter is connected with the inlet conduit 65. When the chamber68 is connected with the drain conduit 66, the piston 70 may be returnedto the starting position by means of hydraulic or pneumatic pressure ona piston face, not shown.

The conduit 65 opens out in a circumferential groove 70 which isconsequently pressurized. Similarly, the drain conduits 66 communicatewith a respective circumferential groove 72, and the outlet conduits 67communicate with a respective circumferential groove 73. A spool 74positioned centrally in the housing is shown in its neutral positionwhere a circumferential flange 75 on the spool exactly bars the groove73 and thus cuts off the outlet conduit 67 topmost on the drawing fromboth the drain conduit 66 and the inlet conduit 65. Similarly, thebottom outlet conduit 67 is cut off from the inlet conduit 65 by meansof another circumferential flange 76 on the spool and is cut off fromthe drain conduit 66 by means of a third circumferential flange 77 onthe spool.

When the spool is moved from its neutral position towards thepositioning means 64, the inlet conduit 65 is put into communicationwith the two outlet conduits 67, and when the spool is moved from itsstarting position towards the first piston 41, the drain conduits 66 areput into connection with the two outlet conduits 67.

Two piston members 78, of which only one is shown in the drawing, abuton the end cover containing the first piston member and project into arespective axially extending bore 79 which communicates continuouslywith the inlet conduit 65 via a pressure conduit 80. Two piston members81 abut on the opposite end cover and project into axially extendingbores 82 in the opposite end of the spool. The piston members 81 and theassociated bores 82 have a substantially larger diameter than the pistonmembers 78 and their associated bores 79.

FIG. 6 shows that a transverse conduit 83 from each bore 82 opens outinto a central longitudinal bore 84 in the spool. The bore 84 isthrough-going in the full length of the spool, and a small pilot spool85 is inserted in the bore. Two circumferential grooves 86 and 87 havebeen so incorporated in the peripheral surface of the pilot spool that aflange 88 positioned centrally between the grooves has a width exactlycorresponding to the width of the transverse conduits 83. The groove 86communicates continuously with the inlet conduit 65 through a pressureconduit 89. Through a drain conduit 90, the groove 87 communicatescontinuously with the drain conduit 66. In the position shown, the pilotspool is in its neutral position, where the central flange 88 cuts offthe transverse conduits 83 from connection with both the pressureconduit 89 and the drain conduit 90.

The electrically controlled positioning means 64 is designed accordingto the linear motor principle, where a movable part 91 carries a numberof windings connected with two freely bendable wires 92. The windingsare positioned between an iron-based core material 93 and a strong,cylinder-shaped magnet 94. When current is passed through the windingsvia the wires 92, the movable part 91 is immediately put into motion,where the direction and speed of movement depends on the direction andintensity of the current. The movable part is associated with a positionsensor 32 which emits signals to the computer concerning the actualposition of the movable part. The movable part 91 is rigidly connectedwith the pilot spool 85 via a rod 95 positioned coaxially with the spool74. A relatively weak compression spring 96 positioned coaxially aroundthe rod 95 abuts the end surface of the pilot spool and an oppositelydirected surface on a centring piece 97 positioned between the end cover43 and the core material 93.

The first piston 41 is rigidly connected with a rod 98 extendingcoaxially with the spool 74 into the central bore 84 of the latter, inwhich bore the rod is centred by means of a trilobate guide member 99.When the camshaft control is inactive, the end of the rod 98 ispositioned at a suitable distance from a corresponding abutment surface100 on the pilot spool, so that the latter is unaffected by the presenceof the rod 98. The computer 16 performs a running monitoring and finesetting of the movable part 91 and thus counteracts the pressure fromthe spring 96. If the electronic control fails, the spring 96 will pressthe pilot spool along to abutment against the rod 98, and simultaneouslythe valve 52 is switched, so that the cam movement is transmittedthrough the second piston 44, the hydraulic conduit 48, the first piston41 and the rod 98, which then positions the pilot spool 85 in thecorrect manner. A compression spring 101 acts on the first piston memberthrough a collar 102 mounted on the rod 98 for movement towards thehydraulic conduit 48. This gives extra security for the first pistonmember 41 rapidly following a downward movement of the other pistonmember 44, when the idler 34 follows the declining side of the cam.

Now, the functioning of the spool valve will be described. As mentioned,there is a continuous pressure in the bore 79, which yields a permanentforce in the upward direction on the drawing on the spool 74. When thepilot spool stands still, it is possible that this upward force willdisplace the spool in the upward direction. If this happens, thetransverse conduits 83 are put into communication with the pressureconduit 89, so that pressurized oil flows into the bores 82. Theconsequent pressure increase in the chamber in front of the pistonmembers 81 acts on the spool with a force which is directed downwardsand forces the spool to occupy the position in which the central flange88 of the pilot spool exactly bars the transverse conduits 83. If thepressure in the bores 82 becomes too great, the spool is moved afraction downwards, thus putting the transverse conduits 83 intocommunication with the drain conduit 90, so that the overpressure in thebores 82 is relieved to the level of balance, where the upward anddownward forces on the spool have the same magnitude.

It is seen from this that the spool 74 will always rapidly set itself inthe position where the central flange bars the transverse conduits 83.As the bores 82 have a larger diameter than the bores 79, there willalways be a resulting force on the spool, if it does not occupy theabove neutral position in relation to the pilot spool. When the pilotspool is displaced in the axial direction of the spool by influencesfrom either the rod 95 or the rod 98, the spool 74 will immediatelyparticipate in this movement for the above reasons. The small mass ofthe pilot spool and the associated rods causes the setting forces on thespool to be extremely small, and makes the spool act very rapidly.

It is, of course, possible to let the first piston 41 act directly onthe spool 74, but this gives a system which acts more slowly and leadsto larger control forces with consequent larger energy deposition in thehydraulic conduit 48.

The camshaft control may be activated for the cylinders individually orsimultaneously for all cylinders, dependent on the kind of failure inthe electronic control system.

The invention may also be used in connection with other types ofelectrically activated positioning means, such as solenoids and stepmotors.

Near the connection for the hydraulic conduit 48 or in said connection,the cylinder 45 for the second piston or the cylinder 42 for the firstpiston may have a compensating volume of a size so that the hydraulicconduits leading to the same kind of cylinder members containsubstantially the same amount of hydraulic oil. This compensating volumemay, for example, be provided by drilling a hole of a larger diameterinto the connecting branch for the hydraulic conduit or by drilling atransverse conduit into the cylinder and plugging the conduit at such adistance from the central outlet conduit of the cylinder that the totalamount of oil between the two pistons is the same for the connectedpairs of pistons.

I claim:
 1. A large two-stroke internal combustion engine (1), inparticular a main engine of a ship, having a hydraulically drivencylinder member, such as a fuel pump (18) or an exhaust valve (13), inwhich the hydraulic drive of the member comprises a driving piston (70)journalled in a hydraulic cylinder (69) which, through a flow passage67, communicates with a spool valve, the spool of which (74) may occupya position where the flow passage (67) communicates with a high-pressuresource (65) for hydraulic oil, and another position where the flowpassage communicates with a low-pressure port (66), and where, duringnormal engine operation, the spool is positionable by means of anelectrically activated positioning means (64) receiving control signalsfrom an engine controlling computer (16), and where, in case of failureof the normal engine control, the spool is alternatively positionable bymeans of a camshaft (23) rotating synchronously with the crankshaft (11)of the engine, characterized in that the spool (74) is associated with afirst piston (41) on which the pressure in a hydraulic conduit (48)acts, said conduit extending to a second piston (44) which may follow acam (26) on the rotating camshaft, and that the hydraulically drivencylinder members (14, 13, 18) associated with each of the enginecylinders are mounted at the pertaining cylinder, whereas the camshaft(23) independently of the positioning of the cylinder members isdisposed at an appropriate shaft drive, such as the crankshaft (11). 2.An internal combustion engine according to claim 1, characterized inthat during normal engine operation, the first piston (41) is preventedfrom transmitting the cam movement to the spool (74).
 3. An internalcombustion engine according to claim 2, characterized in that the secondpiston (44) is lifted free of the camshaft (23) when the engine controlis normal, and that the second piston is brought into contact with a cam(26) on the camshaft, when the latter is to be engaged.
 4. An internalcombustion engine according to claim 1, characterized in that the spool(74) is adapted to follow the movements of a small pilot spool (85)which is controlled by the electrically activated positioning means (64)at normal operation, and alternatively by the movements of the firstpiston (41).
 5. An internal combustion engine according to claim 4,characterized in that the pilot spool (85) is positioned coaxiallyinside the spool (74) and is fastened to a rod (95) which is rigidlyconnected to the movable part (91) of the positioning means and projectsto one side of the spool, and that the first piston (41) is positionedto the other side of the spool and carries a rod (98) which extendscoaxially with the spool to the pilot spool.
 6. An internal combustionengine according to claim 5, characterized in that the first piston (41)with the associated rod (98) is spring-loaded for movement away from thepilot spool (85).
 7. An internal combustion engine according to claim 5,characterized in that the movable part (91) of the positioning meanswith associated rod (95) is spring-loaded for movement towards the firstpiston (41), and that during normal engine operation the positioningmeans (64) overcomes the spring loading.
 8. An internal combustionengine according to claim 7, characterized in that at least some of thepiston-connecting hydraulic conduits (48) leading to the same kind ofcylinder members, are communicating with a respective compensatingvolume of a size so that the hydraulic conduits contain a substantiallyequal amount of hydraulic oil.
 9. An internal combustion engineaccording to claim 1, characterized in that in its active position, thesecond piston (44) abuts the upper side of a rod (33) which on its lowerside carries an idler (34) contacting the associated cam (26), that therod (33) is transversely movable in relation to the longitudinaldirection of the camshaft between an extreme position for use duringrunning of the engine in the normal direction of rotation, and anotherextreme position for use during running of the engine in the oppositedirection of rotation.
 10. An internal combustion engine according toclaim 9, characterized in that the two extreme positions of the rod (33)are adjustable.